The electrodeless discharge lamp is configured that the discharge gas filled in the bulb is activated by high frequency electromagnetic field generated by supplying high frequency current to the induction coil, and ultraviolet light emitted at that time is converted into visible light through fluorescent material. Since the electrodeless discharge lamp apparatus has a configuration that no electrode inside, non-lighting due to deterioration of the electrode may not occur, and thus, it is relatively longevity life in comparison with generic fluorescent lamp.
In a conventional electrodeless discharge lamp shown in Japanese Laid-Open Patent Publication No. 7-272688 or Japanese Laid-Open Utility Model Publication No. 6-5006, for example, uses a bismuth-indium amalgam as a luminescent material. According to this amalgam, it is possible to obtain a higher optical output in a wide range than the optical output at ambient air temperature 25 degrees Celsius, even when ambient air temperature changes. On the other hand, although a high mercury vapor pressure is necessary to realize a high optical output, there, however, is a disadvantage that start-up of the lamp is slower because a time until reaching a temperature value that it is necessary for evaporation of mercury. When the bismuth-indium amalgam was used, a consequence that it is necessary for approximately 1 minute to secure optical output of 60% with respect to optical output at the time of stable lighting was provided.
In contrast, a pure mercury drop is used for the discharge gas to shorten the start-up time in an electrodeless discharge lamp shown in Japanese Laid-Open Patent Publication No. 2001-325920. According to this document, it is mentioned that the optical output was reached 50% of maximum output within two or three seconds after the lamp was activated. This is because the mercury drop needs a shorter time until reaching the temperature value necessary for evaporation than amalgam. When an input power is much larger with respect to a volume of the bulb, or when the ambient air temperature is higher, temperature value of the bulb rises, and mercury vapor pressure falls down adversely, and thus, the optical output falls.
When an amalgam was used as above, variation of optical output is small regardless of variation of ambient air temperature. In contrast, when mercury drop is used, mercury vapor pressure is largely varied corresponding to variation of ambient air temperature, and thus, optical output fall. Accordingly, when mercury drop is used, it is necessary to secure a coldest spot (a portion of a surface of a bulb where temperature value becomes the lowest) so as to control mercury vapor pressure. The temperature is around 35-45 degrees Celsius.
By the way, in an electrodeless discharge lamp shown in Japanese Laid-Open Patent Publication No. 2001-325920, when installation posture thereof is changed, the coldest spot of the bulb is changed. For example, when the lamp is lit in a posture that a ferrule or a cap thereof is disposed upward (hereinafter, it is called “base-up lighting”), a protrusion formed at an apex of the bulb becomes the coldest spot. Alternatively, when the lamp is lit in a posture that a ferrule thereof is disposed downward (hereinafter, it is called “base-down lighting”), a portion of the bulb just above the ferrule becomes the coldest spot. When the volume of the bulb is small, a volume of a portion where discharge occurs becomes relatively larger with respect to the volume of the bulb, so that it is difficult to maintain temperature at the coldest point constant regardless of the posture of installation of the electrodeless discharge lamp. Although temperature at the protrusion of the bulb in the base-up lighting can be controlled by changing a diameter and a height of the protrusion, it is a problem to control temperature at a bulb neck portion in the base-down lighting.